下表是某生物兴趣小组人工孵化蛙卵的实验记录：(1)乙组孵化率低的原因有两个：一是自来水中有氯气等物质对蛙的胚胎有__________作用，二是________低。(2)采取哪些措施可提高乙-八年级生物

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